1. Field of the Invention
The present invention relates to an electrically writable programmable semiconductor memory cell.
2. Description of the Related Art
Japanese Patent Publication No. 3-15345 (Kokoku) discloses an electrically writable programmable semiconductor memory cell which trims resistance by blowing a fuse to adjust the output voltage to the reference voltage.
As shown in FIG. 5, this circuit has an inverter circuit equipped with a load element (depletion MOS transistor) 91, a fuse 92, and a selector switch (enhancement MOS transistor) 93 in sequential serial connection an output line L5 connected to the connection between the load element 91 and the fuse 92 outputs at a high level if the fuse has been blown and at a low level when if the fuse has not been blown. Writing (blowing the fuse) is carried out by applying a high write voltage to the fuse 92 via the load element 91 with the selector switch 93 on, and reading is carried out by applying a lower voltage to the fuse 92 via the load element 91 with the selector switch 93 on.
With the above-mentioned programmable semiconductor memory cell of the prior art, there is a need to reduce the load current in order to reduce current dissipation. Reduction of the load current requires a reduction of the circuit's power supply voltage or an increase in the resistance of the load element, but a reduction in the circuit's power supply voltage--i.e., the voltage of the high-level line for read operations (normally +5 V)--is difficult to achieve because of considerations such as compatibly interfacing with other circuits, output voltage amplitude, and general utility of the device. Therefore, an increase in the resistance of the load element becomes necessary.
However, increasing the resistance of the load element causes a drop in the high-level of the output voltage determined by the ratio of this resistance to the resistance after the fuse has blown, which approaches the threshold voltage (i.e., the boundary voltage between the high-level output voltage when the fuse is blown and the low-level output voltage when the fuse is not blown) and thus a reduction in the voltage margin (i.e., the output voltage amplitude), resulting in a problem of reduced reliability.
Additionally, it is .possible to reduce the above-mentioned power supply voltage without giving consideration to matters such as interfacing with other circuits, but such a reduction in the power supply voltage also causes a reduction in the voltage margin similar to that described above, resulting in reduced reliability.
A second problem of the above-mentioned programmable semiconductor memory cell of the prior art is that because a large voltage is applied to the fusible area of the fuse during read operations, crystal growth occurs on the fused surface of the fuse, and with prolonged use a phenomena such as a decline in fuse resistance and electrical shorting occur, further reducing reliability.